The present invention relates generally to the field of apparatus for measuring the electrical resistance per unit length of a conductor, and more particularly, to an apparatus for measuring the electrical resistance of a conductor without physically contacting the conductor.
Conventional techniques for measuring the electrical resistance per unit length of a conductor (e.g., a wire or cable) entail the use of probes (e.g., 2- or 4-point probes) which are required to be in physical contact with the conductor. However, there are many applications in which it is inconvenient or undesirable to measure the electrical resistance of a conductor by measurement techniques which require such physical contact between the conductor and the test instrument. For example, physically contacting a conductor which is plated or coated with a conductive and/or dielectric protective material may be undesirable because of the risk of damaging or destroying the plating/coating. In this connection, in operations where the electrical resistance per unit length is regulated by controlling the plating thickness on a non-conductive substrate (as in the production of resistive chaff), measuring the electrical resistance per unit length by using a measurement technique which requires physical contact tends to be destructive to the plating and/or substrate material as a result of friction effects.
U.S. Pat. No. 5,083,090, issued to Sapsford et al., discloses a conctactless method for measuring the electrical resistance per unit length of a filament, such as a carbon-coated optical fiber, in which the filament is arranged as the inner conductor of a co-axial transmission line. The electrical resistance of the filament is determined by measuring a propagation characteristic of the co-axial transmission line. The transmission line may be divided into seven sections, with a signal injected on the second section, and a comparison of the resulting signals being used in a feedback loop to control the injected signal frequency in such a way as to hold constant either the relative amplitudes or the relative phases of the signals appearing at the fourth and sixth sections.
In addition to being unduly cumbersome, the Sapsford et al. test apparatus is susceptible to resistance measurement errors due to the adverse influence of physical and/or electrical disturbances in the environment external to the test apparatus which can corrupt the measurements. Further, the Sapsford et al. test apparatus is incapable of measuring the electrical resistance per unit length of low-resistance conductors, e.g., those having resistances less than about 2.times.10.sup.5 .OMEGA./M. Moreover, the Sapsford et al. test apparatus is only capable of measuring the electrical resistance of a conductor for test signal frequencies less than 100 MHz, and is primarily designed to accurately measure the electrical resistance of a conductor for test signal frequencies in a frequency range of between 1 MHz to 4.5 MHz. Thus, the Sapsford et al. test apparatus is incapable of measuring the electrical resistance of a conductor at microwave test signal frequencies.
Based on the above and foregoing, it will be appreciated that there presently exists a need in the art for a an apparatus for measuring the electrical resistance of a conductor without physically contacting the conductor which overcomes the drawbacks and shortcomings of the Sapsford et al. apparatus. The present invention fulfills this need in the art.